Technical Field
Embodiments of the present disclosure relate generally to Phase Locked Loops (PLL), and more specifically to reducing errors due to non-linearities caused by a phase frequency detector of a phase locked loop.
Related Art
Phase locked loops (PLL) are often used to synthesize signals (such as clocks) with a desired frequency. Typically, a PLL contains a phase frequency detector (PFD), a loop filter, a voltage controlled oscillator (VCO) and a divider. The PFD compares a reference signal (reference frequency) at a fixed frequency with a feedback signal (which is a frequency-divided output obtained by frequency-dividing the VCO output in the divider), and generates/activates error signals indicative of a phase difference between the reference signal and the output signal of the VCO. The error signals are filtered by the loop filter, and the output of the loop filter is used to adjust the phase/frequency of the VCO output such that the frequency of the VCO output equals a desired multiple of the frequency of the reference signal.
According to one approach, the PFD provides two error signals as outputs once in every period of the reference signal, with a first error signal being activated if the phase of the reference frequency leads the phase of the feedback signal, and a second error signal being activated if the phase of the reference frequency lags the phase of the feedback signal. The two error signals are used for phase-frequency adjustment of the VCO output as noted above.
The transfer function of a PFD refers to the relationship between the output of the PFD with respect to the input. In general, it is desirable that the transfer function be linear. Non-linearities in the transfer function of a PFD refer to effects due to the implementation and/or operation of the PFD which cause a deviation from a straight line in the transfer characteristics of the PFD. One or more of such non-linear effects may potentially result in errors in the output signal of the PLL, such as for example, increased/additional phase-noise (also termed jitter) in the output signal.
Embodiments of the present disclosure are directed to reducing errors due to non-linearities caused by a PFD of a PLL.
In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.